docs: def getproject_cmd(argv):
      """Print a virtualenv's project directory, if set.
  
      If called without providing a virtualenv name as argument, print the
      current virtualenv's project directory.
      """
      # Parse command line arguments
      parser = argparse.ArgumentParser(
          description="Print an environment's project directory.",
      )
      parser.add_argument(
          'envname',
          nargs='?',
          default=os.environ.get('VIRTUAL_ENV'),
          help=(
              'The name of the environment to return the project directory '
              'for.  If omitted, will use the currently active environment.'
          ),
      )
      args = parser.parse_args(argv)
      # Now, do the actual work
      if not args.envname:
          sys.exit('ERROR: no virtualenv active')
      if not (workon_home / args.envname).exists():
          sys.exit("ERROR: Environment '{0}' does not exist."
                   .format(args.envname))
      project_dir = get_project_dir(args.envname)
      if project_dir is None:
          sys.exit("ERROR: no project directory set for Environment '{0}'"
                   .format(args.envname))
      print(project_dir)